Advection source term refac.

[sbryngelson]

Refactor the nasty advection source routine in RHS.

[codecov]

Codecov Report

Attention: Patch coverage is 48.41270% with 65 lines in your changes missing coverage. Please review.

Project coverage is 43.47%. Comparing base (3e6a84f) to head (9e923d8).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/simulation/m_rhs.fpp	48.41%	56 Missing and 9 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #852      +/-   ##
==========================================
+ Coverage   43.34%   43.47%   +0.13%     
==========================================
  Files          68       68              
  Lines       19902    19770     -132     
  Branches     2384     2377       -7     
==========================================
- Hits         8626     8595      -31     
+ Misses       9825     9727      -98     
+ Partials     1451     1448       -3     

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